This invention relates to an impeller arrangement for use in applying a rotary force to a body derived from a flow of fluid.
A conventional, fixed blade impeller includes a number of blades angled to the fluid flow direction. In such an arrangement, the torque which is generated by the impeller is related to the mass of the fluid diverted by the blades and the angle of deviation of the fluid, which is dependent upon the blade profile and angle.
In a known downhole steerable drilling system, two impellers are mounted on a body, the impellers being designed to rotate in opposite directions, clutch devices being provided to control the transmission of torque to the body. By appropriate control of the clutch devices, the body can be held in a desired geostationary position. The impellers are designed to rotate at high speed, for example at speeds of up to 2500 rpm, generating mechanical friction and viscous drag. Any imbalance between the friction and drag of the two impellers, for example due to particles in the fluid causing jamming, can result in the system becoming unstable. Further, wear of the impellers and associated components can be significant.